Specific colonization of the hypodermis of sorghum roots by an endophyte,Polymyxasp.

Abstract

Abstract A number of clusters of resting spores of an endophyte were found to have colonized the nodal root axes and first order lateral roots of sorghum at the heading stage. The clusters with a diameter ranging 15 to 20 μm consisted of a number of spores with a diameter in the range of 2 to 5 μm. Based on the morphological characteristics, the endophyte was tentatively assigned to Polymyxa sp. The clusters were distributed in the epidermis and the hypodermis, but mostly in the latter. The clusters were absent in the tissues internal to the hypodermis. The number of cells colonized by the clusters in the hypodermis, the hypodermal cell lignification identified by the phloroglucinol-HCl test, and the cortical sclerenchyma development were examined acropetally in three nodal roots which emerged from the third node. The colonized cells accounted for less than about 10% of the total number of the hypodermal cells in the first 8 cm portion, but beyond this part, their number increased and they accounted for 10 to 35%. Hypodermal cell lignification and cortical sclerenchyma development were clearly recognized approximately in the first 9 and 6 cm portion, respectively, but they were not observed beyond this part. Even in this portion, however, no endophyte colonization was observed in any tissues internal to the hypodermis. These findings suggest that in sorghum the hypodermis itself, regardless of cell lignification, may function as a barrier to protect the inner tissues from further colonization by the spore clusters of Polymyxa sp.